1. Field of the Invention
The present invention relates to a cold storage system utilizing natural ice.
2. Description of the Related Art
In Japan, there are many lakes, lagoons and ponds, most of which have an atmospheric temperature sufficiently higher than 0.degree. C. in spring season, so that the ice formed in winter season begins to melt and then entirely melts into water before summer season.
Accordingly, an effective method of utilizing the ice of lake, lagoon and pond has not been found except for utilization as skating rink in some places.
Now, energy resource preservation and environmental protection are a big social issue, and thus an extension of natural energy utilization is an important theme toward the twenty-first century. Ice, being a low temperature and a large heat accumulating density, is suitable for cold storage medium, and its utilization has been researched and developed as a medium for small-sized cold storage system. The ice of lake, lagoon and pond is formed by natural energy, and, because they have a large surface area, a large amount of ice is obtained in winter season. Thus, if at least part of the ice is stored until summer season, and can be used as a cold heat source for cooling and the like, a large energy-saving effect will be expected.
When atmospheric temperature becomes lower than water surface temperature, heat transfers from the water by radiation and convection to the atmosphere. When in winter season atmospheric temperature becomes lower to some extent than 0.degree. C., water surface temperature also becomes 0.degree. C. or less, and water begins to freeze.
When a thin ice is then formed on the water surface, the residual water locating under the ice performs heat exchange through the ice with the atmosphere. In this case, a heat resistance due to ice exists between the residual water and atmosphere, so that the temperature of the residual water becomes higher than that of the surface. However, when atmospheric temperature is fairly less than 0.degree. C., the temperature of the residual water near the ice becomes 0.degree. C. or less and the water ices. Thus, the ice becomes gradually thick, and accordingly the heat resistance between the ice and residual water becomes large and finally the temperature of the residual water does not become 0.degree. C. or less, causing ice formation to stop. Therefore, for lakes, lagoons and ponds in Japan, the ice formed in winter season is often limited to the vicinity of the surface.
On the other hand, when spring season comes, the surface is heated by the sun light or the atmosphere with a raised temperature, and the ice begins to melt gradually from the surface. Since the ice formed in winter season is limited to the vicinity of the surface, it melts in a relatively short period. The ice of lakes, lagoons and ponds disappears completely in summer season when the ice is required as a cold heat source, making the utilization of the ice as a cold heat source very difficult.
Thus, in the conventional lakes, lagoons and ponds, the ice of only the vicinity of the surface is formed, so that the ice cannot be stored until summer season and has not been utilized as a cold heat source by natural energy.